The present disclosure relates to a protection circuit which protects a battery pack from overcurrents and overvoltages using a protection device having heating resistors and fuse elements provided on a circuit board.
As mobile electronic devices such as cellular telephones or notebook PCs have been widely used, the lithium-ion battery market has expanded. These mobile electronic devices typically employ a battery pack having a power supply including one to four lithium-ion batteries connected in series. Such a battery pack may ignite or cause smoke when the lithium-ion battery is overcharged (i.e., becomes under an overvoltage) during recharging, and is thus provided with a protection circuit to avoid overcharges.
This protection circuit is required to protect the batteries from both overcurrents and overvoltages. To this end, a protection circuit is employed which includes a protection device having heating resistors and fuse elements provided on a circuit board, and a sensing device for detecting an overvoltage and switching a current flowing into the protection device. This protection circuit is designed such that the fuse element is melted in an overcurrent condition, while in an overvoltage condition, the sensing device allows a current to suddenly flow through the heating resistor, thereby causing the heating resistor to generate heat by which the fuse element is melted. See, Publication of Japanese Patent No. 2790433.
In recent years, as the market for mobile electronic devices that operate on a large current has been expanded, such a battery pack has come into use that operates at a rated voltage for about 10 serially connected lithium-ion batteries, which is well over previous rated voltages for four or less serially connected lithium-ion batteries.
On the other hand, in the aforementioned battery pack protection circuit, the voltage applied across the heating resistor of the protection device is dependent on the number of serially connected batteries that are included in the battery pack. Accordingly, to ensure that the fuse element of the protection device is melted in an overcharge condition, a lineup of protection devices has to be prepared each of which is provided with a heating resistor having an appropriate resistance value for each number of serially connected batteries. However, now that battery packs have a variety of voltage ratings for four or less to about ten serially connected lithium-ion batteries, this has become problematic due to an increase in costs or price resulting from many different protection devices being produced.
For example, in the protection circuit 1X of FIG. 6 and a protection circuit 1Y of FIG. 7, if protection devices 2A and 2B each include heating resistors 3 and fuse elements 4 are provided on a circuit board, its operable power is 10 to 20 W. One battery 6 within a battery pack 5 has a maximum voltage of 4V, and a voltage sensing IC 8 and an FET 9 are provided as sensing means 7. In this case, the protection devices 2A and 2B have to be prepared such that the heating resistor 3 has the resistance values of Table 1 for each number of serially connected batteries 6 that are included in the battery pack 5.
TABLE 1Number of seriallyResistance valueconnected batteries(Ω)10.8-1.623.2-6.437.2-14 413-26520-40. . .. . .10  80-160
Suppose that in a battery pack 5 having ten serially connected batteries, the protection circuit 1X of FIG. 6 is formed using a 25Ω heating resistor corresponding to a battery pack 5 having four serially connected batteries. In this case, in an overcharge condition, the voltage sensing IC 8 detects an overvoltage across the battery pack 5 resulting in a change in the gate potential of the FET 9. The power consumption W at the heating resistor 3 when a large current flows through the heating resistor 3 is given by the following equation:W=V×V/R=40×40/25=64 WThis thus amounts to 64 W, which is well beyond the operable range of from 10 to 20 W. For this reason, before the fuse element 4 is melted, the heating resistor 3 will burn.
As can be seen from the foregoing example, it is necessary to use a heating resistor 3 of the protection devices 2A and 2B that has a resistance value corresponding to the voltage on the battery pack 5.
On the other hand, a battery pack used in a mobile electronic device operating on a large current requires the protection device to include a large-current fuse element. From this point of view, a lineup of protection devices with fuse elements of various ratings is required, which has become problematic due to an increase in costs or prices of the protection device.